Three pipes A, B and C can fill a tank in 5 hours working simultaneously. The pipe C is twice as faster B and B is twice as fast as A. How much time will pipe A alone take to full the tank ?

35 hrs

The correct answer is option (3) : 35 hrs

Let the pipe A alone fill the tank in t hrs,

Then pipe B alone will fill the tank in $\frac{t}{2}$ hrs

And pipe C alone in $\frac{t}{4}$ hrs.

$∴\frac{1}{t}+\frac{2}{t}+\frac{4}{t}=\frac{1}{5}$

$\frac{7}{t}=\frac{1}{5}$

$t = 35 $

Hence the pipe  A alone fill the tank in 35 hrs.